Production of polycarboxylic acid products



Patented Nov. 27, 1951 PRODUCTION OF POLYCARBOXYIZIC ACID PRODUCTS Robert J. Miller, Berkeley, Calif., assignor to California Research Corporation, San Francisco, Calif a corporation .of Delaware No Drawing. Application October '8, 1947 Serial No. 7783739 1 Claim. 1

CITlis invention relates to a new and improved process for the production of tetracarboxylic acid anhydrides and especially to the production of such anhydrides from aromatic compounds having at least two pairs of ortho-positioned aliphatic substituents.

More particularly, the invention provides anew process for producing tetracarboxylic acid anhydrides, such as pyromellitic anhydride by catalytic conversion of alkyl aromatic hydrocarbons having two pairsof ortho-positioned alkyl groups, such as l,2,3,4 tetra a-lkyl :benzenes; 1,2,4,5 tetra alkyl benzenes, and thelike.

Aryl tetra-carboxylic .acid anhydrideshave not been available commercially at reasonably low cost. Therelative commercial scarcity of such acid anhydrides is due to various factors. Thus, prior processes for obtaining aryl tetra-carboxylic acids have yielded either types of acids which do not form the anhydride readily, if at all, or have given the product :in free .acid rather than anhydride form.

Conversion from the free acid to the anhydride requires a separate dehydration operation with attendant loss and cost. :Other factors frequently responsible for relative scarcity of aryl tetracarboxylic acid anhydrides have been low yields .of these products with formation of correspondingly large amounts of impurities which, because of their similarity chemicaland physical proplerties, create anything but easy purification problems. Expensive reagents heretofore used in the synthesis :of such anhydrides also have added :sig- .nificantly to the .cost of any potential processes.

An object :of this invention is to provide a new and improved process for producing aryl tetrathe ,anhydrides are directly attached to an .aro-

matic nucleus.

The invention has an another object the production of aryl tetracarboxylic acid anhydrides from alkyl aromatic hydrocarbons by oxidation with air or gaseous oxygen.

A further object is to produce pyromellitic an i" hydride form durene and molecular oxygen.

Compounds from which aryl tetracarboxylic acid ,anhydrides may be produced in accordance with this invention are characterized by an aromatic nucleus with an even number of at least p it 1,2,4,5 tetra alkyl l, 2,3,4tetra alkyl Ibenzene benzene Durene is a preferred tetra alkyl benzene, although its 'homologs in which one or more of the above radicals R1, R2, R and "R4 is ethyl, normal propyl, isopropyl, normal butyl, secondary butyl may be utilized. "Higher alkyl radicals are not precluded, but aliphatic groups containing "1 to 4 carbon atoms are preferred since longer chains not only raise the boiling point of the feed to be vaporized, but also increase the 'heatliberated by the reaction with attendant difliculties in temperature control. The invention embraces other feeds in which two 'pairs of-ortho positioned -aliphatic groups are provided, for example:

The invention ,is illustrated by theproduction of pyromelliticanhydride ,f-rom durene or .durenecontainmghydrocarhon fractions or other homolegous ,polyalkyl benzenes' substituted in the 2, 4 and 5 positions. The dureneor durene fraction is vaporized; the vapor phase hydrocarbon mixed with oxygen, .air or other-oxy en-containin .gas and. the resulting gaseous reaction mix ii on,- on; -o

601 O 61120 on CH; o

Some over-oxidation may occur to yield carbon dioxide and additional water, together with minor on a carrier of relatively low surface to volume ratio; for instance, vanadium pentoxide coated on a carrier of silicon carbide or alumina such as Car-borundum or Alundum, respectively. In order to aid in controlling reaction temperatures, particle size of the catalyst mass should be kept relatively coarse. Mesh sizes from 4 to have proven quite satisfactory.

Use of this type of catalyst limits other variables, such as temperatures and air-to-hydrocarbon ratios. It is necessary to operate with an excess of oxygen over that theoretically required to effect the desired oxidation in order to maintain the catalyst in its higher valence states. When using air as the source of oxygen, it has been found that this process can be operated with amounts of ring rupture derivatives easily sepa-' rable from the pyromellitic anhydride.

mellitic anhydride are formed in good purity and can be separated readily from the gas stream issuing from the reactor. Proper control of conditions is required to secure the most favorable results. This requires limiting such variables as reactor temperature; air rate, feed rate, air-tohydrocarbon ratio, nature of the catalyst, amount of catalyst, nature of the catalyst carrier, and particle size of the carrier.

Operable temperature range for effecting the oxidation is limited at the lower extremity by the activity of the preferred catalyst. It is preferred to'use as catalyst an oxide of vanadium. Below about 700 F., vanadium oxide is incapable of being reoxidized from its lower to its higher valence states by the use of air alone, and since its function as a catalyst depends upon its ability to freely interchange its valences, temperatures below about 700 F. are not practical. However, other catalysts are also effective in this oxidation which are capable of regeneration from lower to higher oxidation states with air or oxygen at temperatures considerably below this; for example, tin vanadate is most effective in the neighborhood of 550 F. Upper temperature limit is governed by the degree to which the heat of the oxidation reactions can be controlled. At temperatures above about 14'70 F., the catalyst mass will fuse and tend to plug the reactor tubes. Above about 1200 F. the pyromellitc anhydride becomes unstable and the resultant yields of product are lowered. For these reasons, it is desirable to make every effort to disperse the heat of the exothermic oxidation reactions.

It has been found that satisfactory temperature control can be obtained by carrying out the oxidation in a reactor resembling a heat exchanger in which the tube bundle is packed with catalyst and constitutes the reactor, and a suitable coolant,

such'as a molten salt or boiling mercury, is circulated through the shell. By keeping the diameter of the tubes sufl'iciently small, say, inch inside diameter, satisfactory temperature control has been obtained With such an arrangement, it has been found that this process can be effected with reactor temperatures (bath is one composed primarily of oxides of vanadium an air-to-hydrocarbon ratio between 20 and 1000 by weight, but preferably to keep it between 30 and 200 to obtain best yields commensurate with high throughtput.

Space' rate isnot particularly critical over a wide range. It is preferred to consider it in terms of air rate and feed rate for a given catalyst tube. For instance, with other variables held constant, the quantity of catalyst in a reactor tube can be varied from about 100 cc. down to 25 cc. without materially affecting yields and conversions. This is because essentially the entire oxidation occurs in the immediate neighborhood of the hot spot, a region varying in depth between a fraction of an inch and several inches. Use of more catalyst is desirable as a safetyfactor in case of fluctuation in catalyst activity.

In a reactor consisting of a single three-foot tube of inch inside diameter immersed in a cooling bath and packed with to cc. of catalyst, it is preferable to use an air rate between 40 and '70 mols per hour, and a feed rate between 0.05 and 0.3 mol per hour.

Alternatively, a so-called fluid catalyst technique and process may be utilized, salient features of which are use of low gas velocities and a powdered catalyst. To avoid undue loss in catalyst activity, it is preferred to use a pulverulent vanadium oxide catalyst in which the support is vanadium metal. (However, vanadium oxide alone or vanadium oxide with other supports are operable.) Desirably, the vanadium oxide is indigenous to the vanadium support.

The fluid catalyst technique is one in which powdered catalyst is handled like a fluid and is well known for catalytic cracking in the petroleum industry. Such types of processes and fluid powder catalyst handling mechanisms are disclosed in Petroleum Refiner, 1947 Process Hand Book, Section I (Apr. 1947), pages -153.

Example 1 A three-foot long stainless steel tube of inch insidediameter and jacketed with a bath stream of air flowing at 53'mols per hour, already preheated to 280 F., and the two components flowed concurrently through the vaporizer at 280 F. so that at the point of exit from the vareactor.

ture ready for feeding to the-preheat section of the This consisted of the -volume of the reactor tube above the catalyst, which was packed with uncoated silicon carbide and heated to the reactor temperature, e. g., .850 F. The gaseQus mixture than passed over the catalyst where a hot spot of 1050" F. developed four inches from the top of the catalyst. Vapors issuing from the reactor passed into two parallel air-cooled condensers, consisting of 30-inch long glass tubes-of 2 inch inside diameter containing four wire screen baflies spaced equidistant along each tube. The pyromellitic anhydride and any other products of low volatility were collected in these condensers as solids. Waste gases from the tubes consisted of spent air, water vapor, CO and C02 from the oxidation, traces of other intermediate oxidation products, and about 5% of the pyromellitic anhydride that had been formed. Analysis of condenser products after operation of the unit for one hour showed 22.4 g. of crude pyromellitic anhydride to have been formed from 31.3 cc. (25.3 g.) of durene, or an 89.0 weight per cent yield. The neutral equivalent of this crude product was 55.5 (theoretical, 54.5), indicating a purity of 98.2%.

Example 2 The reactor described in Example 1 was packed with 50 cc. of catalyst instead of 100 cc. All, other conditions and equipment were similar, except that durene was pumped at 28.8 cc. per hour. A 93.7 weight per cent yield of crude pyromellitic anhydride of 93.5% purity by neutral equivalent was formed.

Example 3 Conditions and equipment were duplicated in Example 2, except that the bath temperature was increased to 890 F., and durene was added at 29.7 cc. per hour. A 98.5 weight per cent yield of crude pyromellitic anhydride of 92% purity by neutral equivalent was formed.

Example 4 Conditions and equipment were the same as in Example 1, except that 100 cc. of 4 to 6 mesh Alundum coated with vanadium pentoxide were used as catalyst in place of Carborundum coated catalyst. Durene was pumped at 30 cc. per hour. A 90.0weight per cent yield of crude pyromellitic anhydride of 94.4% purity by neutral equivalent was formed.

Example 5 Conditions and equipment were the same as in Example 4, except that bath temperature was held at 850 F., and liquid durene was pumped at 20.7 cc. per hour. A weight yield of 102% of crude pyromellitic anhydride of 94% purity by neutral equivalent was formed.

Example 6 Example 2 was duplicated, except thatdurene was pumped at 48.3 cc. per hour. An 84.7 weight 6 peracentwield ofcrude pyromel-liztic. anhydride :91 98. 1% purity-by neutral equivalentwas obtained.

' Example Example 4 was duplicated except that'air was fed at 63 mols per'hour, instead of 53, and durene was pumped at 15 .cc. per hour with a bath temperature of 890F. A 102.5 weight per cent yield ore-1.3% pyromellitic anhydride was collected.

Example 9 Example 10 The same equipment and catalyst as in Example 4 were used except that 1,4 dimethyl, 2,5 di-ethyl benzene was charged in place of durene. The material charged possessed the following characteristics: R. 1., 1.5092 at 20 0.; D4 0.8845; F. P., 32.5 C.; and B. P., 227 C. The air rate was maintained at 53 mols per hour and the hydrocarbon was fed in at 14.4 cc. per hour, while the reactor temperature was maintained at 850 F. A 38.2 weight per cent yield of pyromellitic anhydride of 92.3% purity by neutral equivalent was formed.

Although the product usually will be desired in the anhydride form and should be recovered as such, it is to be understood that the invention is not avoided by recovering the tetracarboxylic acid product in the form of the free acid rather than as the anhydride. In the appended claim, the terms tetracarboxylic acid product, polycarboxylic acid product, as distinguished from tetracarboxylic acid, are used in a generic sense, embracive of the free acid and anhydride forms.

Pyromellitic product may be recovered as the anhydride by condensation from combustion gases or by washing with a solvent. Upon washing the combustion gases with water, for example, the free acid is obtained and may be crystallized from the solution in the form of the dihydrate, i. e., C6H2 (COOHM'2H2O. Water of crystallization may be removed by heating.

In the oxidation of durene fractions according to this invention, it should be noted that other hydrocarbon impurities are over-oxidized, at least to the point where they are readily separable. Thus, methyl groups are burned ofi entirely or the ring is ruptured, or both, until byproduct impurities differ in major respects from the desired product. This greatly simplifies subsequent final purification. Other impurities, if any, may be removed in suitable fashion. The preferred method of initial purification as herein illustrated is by selective crystallization or condensation of the tetracarboxylic acid anhydride from the mixture of reaction gases.

Obviously, many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and only such limitations should be imposed as are indicated in the appended claim.

I claim:

A process for oxidizing durene to pyromellitic I anhydride, which comprises forming a gaseous mixture of air and durene, the weight of air in said mixture being from 30 to 200 times the weight of durene, contacting the gaseous mixture with a vanadium oxide catalyst disposed on an inert solid support at a temperature in the range 850 to 1050 F. and at a rate such that 0.05 to 3.0 moles of durene are contacted with 50 to 100 cc. of catalyst in one hour.

ROBERT J. MILLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES 10 Marek-Hahn: Catalytic Oxidation of Organic Compounds, 1932, page 395. 

